The present invention relates in general to current mirrors and in particular to a current mirror utilizing a small capacitance for stability compensation.
Current mirrors (also known as current repeaters) are widely used in integrated circuits implementing operational amplifiers, digital-to-analog converters, and the like to produce an output current which is substantially equal in magnitude to an input current. One well known current mirror employs first and second matching bipolar transistors having interconnected bases and having emitters respectively coupled through matching first and second resistors to a common potential source. A collector voltage of the first transistor is fed back to the transistor bases through a unity gain amplifier to form a feedback loop. When an input current source is connected to the collector of the first transistor, the feedback loop adjusts the base voltage so that the collector of the first transistor carries the input current. Since the bases of the first and second transistors are interconnected, and since the first and second resistors as well as the transistors are matched, the second transistor produces an output collector current substantially equal in magnitude to the input current.
To prevent circuit instability, the feedback loop is altered by connecting a compensating capacitor between the collector of the first transistor and the common potential source, the capacitor being sized to adjust the frequency, at which the absolute value of the open loop gain of the feedback loop is unity, to less than the short-circuit current gain-bandwidth product (f.sub.T) of the first transistor. (The parameter f.sub.T is an inherent characteristic and is defined as the frequency at which the short-circuit common-emitter current gain of the transistor attains unity magnitude.) Unfortunately, when the short-circuit current gain-bandwidth product of the first transistor is low (as for example when it is implemented in the form of a lateral pnp transistor), the size of the compensating capacitor needed to insure circuit stability may exceed the maximum capacitance which can be realized on an integrated circuit or may require much more chip area than can be economically justified, particularly when there is no thin oxide dielectric layer on the chip for allowing higher capacitances to be realized. The requirement for a large capacitance is aggravated as the collector currents of the transistors become large, increasing the transconductance of the transistors. Therefore the compensating capacitor must be implemented in the form of a more complex and expensive discrete component connected to external terminals of the chip, consuming package pins and increasing packaging cost. What is needed is a stable, compensated current mirror which may be implemented with transistors having a low current gain-bandwidth product, without requiring a large, discrete compensating capacitance.